Many modern automated transmissions used in medium and heavy duty truck applications include a so-called "hold mode" feature that allows the vehicle operator to effectively lock the automated transmission in any desired gear, and which may further include the capability to permit manual upshifts and downshifts during hold mode operation. The hold mode feature is intended, at least in part, to provide added flexibility and driver control of the vehicle under low road speed and high engine load conditions by allowing the driver to override transmission shift point logic. Examples of vehicle operating conditions wherein such hold mode operation is beneficial include, but are not limited to, maneuvering away from loading docks, traversing intersections, climbing positive grades, operating in extreme terrains, and the like.
While the hold mode feature of many automated transmissions have been generally well received and widely implemented in the trucking industry, this feature has several drawbacks associated therewith. For example, the hold mode feature allows the vehicle operator to select hold mode operation at will and resultantly disable automated shift point control logic as desired. Under certain vehicle operating conditions, such as low engine load conditions, vehicle operation at a numerically lower gear than would otherwise be selected by the shift point control logic results in unnecessarily high engine rpm operation and degraded fuel economy. In vehicles having manual transmissions, a so-called "geardown protection" feature may be implemented to limit vehicle speed when the vehicle operator has not selected the numerically highest transmission gear, thereby encouraging efficient shifting. An example of one such geardown protection feature is described in U.S. Pat. No. 5,564,999 to Bellinger et al., which is assigned to the assignee of the present invention. At present, however, no systems are known which provide a hold mode override feature that effectively discourages hold mode operation when a legitimate need therefore does not exist.
What is therefore needed is a system for overriding the hold mode feature of an automated transmission during vehicle/engine operating conditions where hold mode operation is undesirable, thereby encouraging or forcing more efficient engine and vehicle operation.